There are many situations in which it is desirable to replace hot, humid, polluted, or stale air within a building. Sometimes air replacement is desirable for the comfort of the occupants, such as when a building that does not have air conditioning is oppressively hot. Sometimes air replacement is desirable for the health and safety of the occupants, such as when an activity is being carried out which creates fumes.
One would normally use a fan to create air movement. Fans are relatively inexpensive and are available in a variety of sizes, so as to meet most needs. However, fans only cause air movement, not replacement. Air will just circulate within the limits of the fan. Furthermore, in the workplace, a fan positioned in a travel area such as a doorway or a hallway would interfere with the normal operation of businesses by blocking ingress and egress.